In an electronic circuit for inputting a radio frequency signal, for example, the IC (integrated circuit) of a radio circuit is provided with a protection circuit for preventing the breakage of a circuit due to electro-static discharge. FIG. 10 is a circuit diagram showing an example of the configuration in which an ESD protection circuit is provided at the radio frequency signal input part of the reception block of a radio circuit.
A low noise amplifier (LNA) 102 is connected to the latter stage of a terminal part (pad) 101 for inputting a signal from the outside. In the example of FIG. 10, in order to prevent the amplitude of the input signal from increasing to an allowable voltage value or more due to electro-static discharge, an ESD diode circuit 103 acting as an ESD protection circuit is inserted between the terminal part 101 and the low noise amplifier 102.
In the ESD diode circuit 103, two diodes 103a, 103b are connected in series, the cathode of the diode 103a is connected to a power supply, and the anode of the other diode 103b is connected to the ground (GND). When the input signal changes to the positive voltage side due to electro-static discharge, the diode 103a is switched into an ON state to thereby flow a current into the diode 103a, thereby preventing the input signal amplitude from increasing to the power supply voltage or more. In contrast, when the input signal changes to the negative voltage side, the diode 103b is switched into an ON state to thereby flow a current into the diode 103b, thereby preventing the input signal amplitude from reducing to the ground voltage or less.
However, each of the diodes 103a, 103b constituting the ESD diode circuit 103 has a parasitic capacitance. In the case of providing the ESD diode circuit 103, the parasitic capacitance of the diode could be a factor for degrading the transmission characteristics of a reception signal. In a radio circuit treating a microwave signal of several GHz as an input signal, the technique of reducing the parasitic capacitance of the ESD diode circuit has been investigated in order to suppress the influence on the transmission characteristics.
The higher the frequency becomes, the larger the influence of the parasitic capacitance becomes. For example, in the case of treating a millimeter wave signal of several tens GHz which is ten times or more of the microwave band, the influence of the parasitic capacitance increases. Thus, in the radio circuit of the millimeter wave band, it is difficult to eliminate the degradation of the transmission characteristic due to the parasitic capacitance.
As a solution for eliminating the problem caused by the parasitic capacitance of the diode, there has been proposed an ESD protection circuit using transmission lines (see Patent Literature 1). FIG. 11 is a circuit diagram showing a first example of the ESD protection circuit described in Patent Literature 1, and FIG. 12 is a circuit diagram showing a second example of the ESD protection circuit described in Patent Literature 1.
The first example of FIG. 11 includes a transmission line 154 for connecting a terminal part (pad) 151 and a grounding point, a transmission line 155 for connecting the input of an on-chip circuit (low noise amplifier, for example) 157 and a bias power supply circuit 168, and a capacitor 156 for connecting the terminal part 151 and the input of the on-chip circuit 157.
Since the impedance of the transmission line 154 is high in the millimeter wave band, a signal passes toward the on-chip circuit 157 from the terminal part 151. On the other hand, a signal generated by electro-static discharge has a frequency sufficiently lower than the millimeter wave band. Since the impedance of the transmission line 154 becomes low with respect to an electro-static discharge signal of the low frequency, the protection circuit shows characteristics similar to that in the case where the signal path and the grounding point is short-circuited therebetween. Thus, the transmission of the electrostatic discharge signal to the on-chip circuit 157 can be suppressed.
The second example of FIG. 12 shows a configuration in which both the transmission line 154 and the capacitor 156 of FIG. 11 are not provided. In this example, the ESD diode circuit 103 and a resistor 159 are connected between the transmission line 155 and the bias power supply circuit 168, one end of a capacitor 158 is connected to a connection point between the transmission line 155 and the ESD diode circuit 103, and the other end of the capacitor 158 is grounded.
Since the impedance of the transmission line 155 is high in the millimeter wave band, the influence on the signal path due to the parasitic capacitance of the ESD diode circuit 103 can be suppressed. Since the impedance of the transmission line 155 becomes low with respect to the electro-static discharge signal of the low frequency, the protection circuit shows characteristics similar to those in the case where the signal path and the ESD diode circuit 103 are connected. Thus, the signal amplitude can be suppressed by the ESD diode circuit 103 when electro-static discharge occurs.